Method of covering resistor bead



May 10, 1966 M. SAPOFF 3,249,988

METHOD OF COVERING RESISTOR BEAD Filed Feb. 27, 1962 FIG. 5

INVENTOR. Wire-e 54 0- various ways.

3,249,988 METHOD OF COVERING RESISTOR BEAD Meyer Sapotf, West Orange, N.J., assignor to Victory Engineering Corporation, Springfield, N.J., a corporation of Delaware Filed Feb. 27, 1962, Ser. No. 175,953

' 1 Claim. (Cl. 29-15563) This invention relates to a method and means for protecting thermistors by enclosing the thermistor head in a very thin covering of glass. The conductors to the thermistor are also covered and supported by a portion of the glass covering. The invention has particular reference to the method of heating a thin glass tube in order to completely cover a head with a uniform thin insulating covering.

Therrnistor beads must be covered with some sort of insulating covering so that they may be protected from chemical attack by materials which surround them. The covering must be an insulating layer so that the conductors leading to the thermistor and portions of the thermistor itself are not short-circuited by ambient fluids. Prior art devices of this nature have been protected in Some coverings were applied by painting or spraying plastic coverings and then permitting the applied material to dry or to harden with a heat treatment. Such coverings have not proved satisfactory because the thickness of the covering varied considerably, there were voids and air bubbles in the covering layer, and fissures and cracks often appeared in the covering after the thermistors were used for a short time. The present invention deposits an extremely thin film of glass on the thermistor which not only protects it and provides a rapid response to ambient conditions but also serves as a support for the lead-in conductors.

The method and means of covering very small beads of resistant material has been applied to thermistors which have a coefficient of resistivity which varies widely with the temperature. However, similar coatings may be applied to other small circuit elements such as ordinary resistors, varistors, and other types of circuit elements which require more than ordinary protection.

One of the objects of this invention is to provide an improved method of covering a thermistor which avoids one or more of the disadvantages and limitations of prior art methods.

Another object of the invention is to cover a thermistor head with an extremely protective layer of glass.

Another object of the invention is to reduce the thickness of thermistor coatings.

.Another object of the invention is to increase the response of thermistors to changes in ambient temperature.

Another object of the invention is to eliminate voids and air bubbles in thermistor coverings.

Another object of the invention is to eliminate fissures,

' cracks, and holes in the protective coverings of thermistors.

The invention comprises the method of covering a thermistor bead having at least two conductors sea-led into the body of the head. The method comprises the followings steps: Inserting the bead and conductors into a thin-walled glass tube, applying heat to the glass tube until the glass becomes soft and shrinks around the bead and portions of the conductors, and then removing the excess soft glass.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings.

FIGURE 1 is a cross-sectional view of one form of thermistor with a prior art coating.

United States Patent delay.

3,249,988 Patented May 10, 1966 ice FIGURE 2 is a cross-sectional View of a thermistor with another form of prior art coating.

FIGURE 3 is a cross-sectional view of a thermistor inserted into a thin-walled glass tubing.

FIGURE 4 is a cross-sectional view showing another step in the process of depositing a coating on a thermistor.

FIGURE 5 shows the completed thermistor made in accordance with the method of this invention.

Referring now to FIGURE 1 a thermistor is shown which has been coated by spraying a plastic solution onto the exterior surface. The thermistor 10 has two conductors 11 and 12 embedded in the thermistor bead. The enlarged drawing shows several cracks in the coating 13 and areas having no protection from the covering.

The thermistor shown in FIGURE 2 is another form of prior art development where the covering 13 has been deposited by repeated dipping into a solution and then permitting the covering to dry. This method of coating results in a very thick film which impedes the flow of heat from ambient fluids to the thermistor bead 10 and results in a poor response characteristic. Coverings of this nature must be of a certain thickness in order to protect the entire thermistor surface.

FIGURES 3, 4 and 5 show successive stages during the manufacture of a thermistor coating as described herein. In FIGURE 3 the thermistor 10 together with conductors 11 and 12 are inserted into a glass tube 14 having a relatively thin wall. Then, as shown in FIG- URE 4, heat is applied to the thermistor bead and its covering by means of a flame 15, or the thermistor and glass tube assembly may be put into a suitable furnace. As the walls of the glass tube become plastic, the tube ends are separated so that the tube wall 14A surrounding the thermistor becomes quite thin, this dimension being variable but under control of an operator.

Because of the natural tendency of molten glass to shrink or contract, the walls 14A are compressed around the thermistor body in secure contact with the surface of the bead. As the heating continues one end portion 14B is pulled away from the thermistor and the tube opening is sealed shut. The other portion of the glass tube 14C is heated and permitted to collapse around the conductors 11 and 12, enclosing them in a hermetic seal and insulating the conductors from each other. The excess glass at 14B is then removed and the glass further heated to seal the bead in the glass.

FIGURE 5 shows the finished product with the thermistor 10 enclosed by a thin film of glass 14A which protects it from ambient fluids but is thin enough for passing heat readily from a fluid to the thermistor body. It is evident from this figure that the conductors 11 and 12 are held rigidly by the collapsed glass 14C. The supporting structure may also be employed to hold conductor welds 17 and 18 in the event that thicker lead-in conductors are to be joined to the thinner conductors inserted into the bead.

The result of the above described process is the formation of a covering for a thermistor which is not only rugged continuous and chemically inactive but is also thin enough to transmit heat without an appreciable time Having thus fully described the invention, what is claimed as new and desired to be secured by Letters Patent of the United States, is:

The method of covering a resistor bead having at least two electrical conductors sealed into the body of the bead comprising the followings steps: inserting the bead and conductors into a thin walled glass tube with the conductors lying axially within the tube, applying heat to the glass tube adjacent to the bead by means of a flame until the glass becomes soft, pulling one end portion of the tube until'the glass is stretched around and surrounds the bead and adjacent portions of the conductors in close contact therewith, continuing the pull until the excess portion of the tube at the bead end is severed from the bead surrounding portion, heating the opposite end portion of the tube until it collapses around the conductors to insulate them from each other, removing the remaining excess tube portion of glass of the assembly and thereafter reheating the assembly to provide an even coating over the entire surface.

References Cited by the Examiner UNITED STATES PATENTS 845,413 2/1907 Haagn 29155.63 X 2,263,601 11/1941 Wendler 29155. 63

4 Chrisensen et a1. 29-15563 Morse 65-56 X Brown.

Weiller 338. 28

Pies 29-155.63

Math 29-15563 Bronson et a1. 29-15563 Kohring 29-15563 Bruen 65--S9 X Examiners.

15 J. T. POWELL, J. w. BOCK, W. 1. BROOKS,

Assistant Examiners. 

